ABSTRACT The overall goal of this proposal is to evaluate cartilage composition of patients with hip osteoarthritis (OA) using advanced MRI techniques, and determine their relationship to development of cartilage lesions, patient function, and changes in hip kinematics and kinetics during functional tasks. The central hypothesis is that early changes in cartilage biochemistry and altered hip biomechanics are associated with progression of hip OA defined by worsening patient-reported outcomes and higher cartilage lesion scores. To accomplish this we need to establish the cross-sectional relationship of these metrics and natural history of hip OA over three years with the following specific aims: Aim I) To characterize hip cartilage T1? and T2 relaxation time, their spatial heterogeneity and anatomical location, and association of hip geometry (i.e. alpha angle, acetabular depth, etc), as a function of (I.A) KL grade and (I.B) annual follow-up time (Years 0-3); Aim II) To determine the relationship between hip cartilage composition (measured by MR T1? and T2 relaxation time, spatial heterogeneity, their anatomical location - Aim I outcomes) and disease progression as measured by (IIA) hip joint morphology (see Fig. 1- semi-quantitative MR grading1, adjusted for hip geometry such as alpha angle) and (IIB) patient reported outcomes (as determined by HOOS15); and Aim III) To determine the relationship between hip cartilage composition (measured by MRI T1? and T2 relaxation time, Aim I outcomes), and sagittal plane hip kinematics and kinetics during functional tasks (peak hip flexion angle and peak hip extension moments during gait, stairs, and sit-to-stand). A total of 144 hips across a range of OA severity will be included in the study. Advanced quantitative imaging, motion analysis, and functional testing will be performed annually over three years (baseline, 1, 2, and 3 year follow-up). This comprehensive evaluation of the associations of hip cartilage relaxation times, lesion prevalence, and biomechanics is the vital first step to determine the natural history of hip OA using non-invasive assessment. This information forms the crux of what is needed to design and assess the effectiveness of conservative (gait retraining, muscle strengthening), disease modifying drugs, and surgical (femoral and pelvic osteotomy, etc.) interventions at slowing or reversing the disease process.